This invention relates to a process for coating reflector-type fluorescent lamp envelopes and the resulting lamp and, more particularly, to an improved process for coating envelopes for reflector-type fluorescent lamps wherein only one lehring step is used to remove binder from the applied plural coatings.
Reflector-type fluorescent lamps have been known for many years and are used for applications where the light from the lamps is to be directed downwardly. Such lamps have been fabricated by applying over the envelope a finely divided white-appearing material which extends radially around only a predetermined proportion of the interior circumferential surface of the envelope to leave a reflective-material-uncoated longitudinal aperture through which the generated light may pass. Over the entire interior surface of the envelope is coated a light-generating finely divided phosphor which covers over both the reflective material coating and the remaining longitudinal aperture. Representative lamps have utilized an aperture which subtends an angle of about 130.degree., with the reflective coating material covered over the remaining 230.degree. of the envelope. The best material for forming the reflector is finely divided titania which functions very effectively because of its state of division and its extremely high index of refraction.
In fabricating such lamps in accordance with the best practices of the prior art, the titania is first applied by flushing a slurry or so-called paint of the titania over a limited section of the envelope and then lehring the envelope at a somewhat limited temperature, such as 500.degree. C to 550.degree. C for 1 minute, to partially remove the binder material therefrom. The ends of the envelopes are then cleaned in order to remove reflective material therefrom to permit the lamp cathode carrying mounts to be later sealed in the cleaned envelope ends during the lamp fabrication. After the end cleaning operation, the envelopes are then fully lehred, such as at a temperature of 650.degree. C for 1 minute, in order to completely remove the binder material from the applied titania coating. The phosphor paint is thereafter applied over the entire interior envelope surface, the envelope is lehred again to volatilize binder from the applied phosphor, the applied phosphor cleaned from the end portions of the envelope to permit the mounts to be sealed therein, and lamp fabrication then completed.
The foregoing three lehring operations, two at a relatively high temperature and one at a somewhat restricted temperature, have caused the envelopes to shrink in their longitudinal dimension and a 4 foot (122 cm) fluorescent lamp envelope can shrink in longitudinal dimension from 20 mils (0.5 mm) to 50 mils (1.5 mm). Since a fluorescent lamp must be very carefully controlled with respect to its length dimension in order to fit into the fixtures, this has initially necessitated fabricating extra long envelopes where reflector-type lamps were to be made, creating problems of fabrication, storage and inventory. In addition, the three separate lehring steps have constituted a substantial expenditure in labor and time.
The foregoing separate lehring operations have been required to be able to end clean the applied titania from the coated envelope since even though the titania is a relatively refractory material, at the indicated lehring temperatures of about 650.degree. C it will incipiently fuse to the glass envelope thereby making end cleaning extremely difficult. For this reason, the initial reduced lehring temperature of about 500.degree. C to 550.degree. C is needed to enable the envelopes to be end cleaned, with the binder thereafter completely removed by relehring at an increased temperature.